Oh sure, on the slight coincidence that you gemmed/enchanted exactly how I want, then it isn't wasted. But how likely is that?

Consider the opposite case, you don't get it all right. I'm not going to pay extra for the enchant that I don't want. I'll just buy the one with a smarter seller. If I do buy from you it will be because you are the one that ate the cost of the enchant. Either way, someone loses.